Polyester membranes for aromatics/saturates separation

ABSTRACT

Polyester membranes show good selectivity for aromatics vs. saturates. Modified, e.g., crosslinked, polyesters and their blends can also be used.

BACKGROUND

The use of membranes to separate aromatics from saturates has long beenpursued by the scientific and industrial community and is the subject ofnumerous patents.

U.S. Pat. No. 3,370,102 describes a general process for separating afeed into a permeate stream and a retentate stream and utilizes a sweepliquid to remove the permeate from the face of the membrane to therebymaintain the concentration gradient driving force. The process can beused to separate a wide variety of mixtures including various petroleumfractions, naphthas, oils, hydrocarbon mixtures. Expressly recited isthe separation of aromatics from kerosene.

U.S. Pat. No. 2,958,656 teaches the separation of hydrocarbons by type,i.e., aromatic, unsaturated, saturated, by permeating a portion of themixture through a non-porous cellulose ether membrane and removingpermeate from the permeate side of the membrane using a sweep gas orliquid. Feeds include hydrocarbon mixtures, e.g., naphtha (includingvirgin naphtha, naphtha from thermal or catalytic cracking, etc.).

U.S. Pat. No. 2,930,754 teaches a method for separating hydrocarbons,e.g., aromatic and/or olefins from gasoline boiling range mixtures, bythe selective permeation of the aromatic through certain non-porouscellulose ester membranes. The permeated hydrocarbons are continuouslyremoved from the permeate zone using a sweep gas or liquid.

U.S. Pat. No. 4,115,465 teaches the use of polyurethane membranes toselectively separate aromatics from saturates via pervaporation.

The present invention relates to a process for the separation ofaromatics from saturates.

Compared to distillation, membrane permeation can lead to considerableenergy savings. A membrane can separate a mixture of aromatics andsaturates, e.g., a heavy cat naphtha, into a high-octane, mainlyaromatic permeate and a high-cetane, mainly saturated retentate. Bothpermeate and retentate are more valuable than the starting heavy catnaphtha.

SUMMARY OF THE INVENTION

The present invention is a method for separating mixtures of aromaticsand non-aromatics into aromatic-enriched and non-aromatic-enrichedstreams by contacting the aromatics/non-aromatics mixture with one sideof a polyester membrane, and selectively permeating the aromaticcomponents of the mixture through the membrane.

BRIEF DESCRIPTION OF THE DRAWINGS

The sole FIGURE shows the toluene/isooctane selectivity and permeabilityfor a membrane according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention is a method for the separation of aromatics fromsaturates using polyester membranes. Several polyesters are commerciallyavailable, e.g., polyethylene terephthalate, polybutylene terephthalate,and polyethylene terephthalate/cyclohexane-dimethanol terephthalate(PETG). That is, PETG is a polyethylene terephthalate in which enoughethylene glycol units have been replaced with 1,2-cyclohexanedimethanolunits to make it amorphous.

In the present invention, membranes are used to separate a mixture ofaromatics and non-aromatics into an aromatic-enriched fraction and anon-aromatic-enriched fraction.

The membranes are useful for the separation of aromatics from saturatesin petroleum and chemical streams, and have been found to beparticularly useful for the separation of large substituted aromaticsfrom saturates as are encountered in heavy cat naphtha streams. Otherstreams which are also suitable feed streams for aromatics fromsaturates separation are intermediate cat naphtha streams boiling at93°-160° C., light aromatics content streams boiling in the C₅ -150° C.range, light catalytic cycle oil boiling in the 200°-345° C. range aswell as streams in chemical plants which contain recoverable quantitiesof benzene, toluene, xylenes (BTX) or other aromatics in combinationwith saturates. The separation techniques which may successfully employthe membranes of the present invention include perstraction andpervaporation.

Perstraction involves the selective dissolution of particular componentscontained in a mixture into the membrane, the diffusion of thosecomponents through the membrane and the removal of the diffusedcomponents from the downstream side of the membrane by the use of aliquid sweep stream. In the perstractive separation of aromatics fromsaturates in petroleum or chemical streams (particularly heavy catnaphtha streams) the aromatic molecules present in the feedstreamdissolve into the membrane film due to similarities between the membranesolubility parameter and those of the aromatic species in the feed. Thearomatics then permeate (diffuse) through the membrane and are sweptaway by a sweep liquid which is low in aromatics content. This keeps theconcentration of aromatics at the permeate side of the membrane film lowand maintains the concentration gradient which is responsible for thepermeation of the aromatics through the membrane.

The sweep liquid is low in aromatics content so as not to itselfdecrease the concentration gradient. The sweep liquid is preferably asaturated hydrocarbon liquid with a boiling point much lower or muchhigher than that of the permeated aromatics. This is to facilitateseparation, as by simple distillation. Suitable sweep liquids,therefore, would include, for example, C₃ to C₆ saturated hydrocarbonsand lube basestocks (C₁₅ -C₂₀).

The perstraction process is run at any convenient temperature,preferably as low as possible.

The choice of pressure is not critical since the perstraction process isnot dependent on pressure, but on the ability of the aromatic componentsin the feed to dissolve into and migrate through the membrane under aconcentration driving force. Consequently, any convenient pressure maybe employed, the lower the better to avoid undesirable compaction, ifthe membrane is supported on a porous backing, or rupture of themembrane, if it is not.

If C₃ or C₄ sweep liquids are used at 25° C. or above in liquid state,the pressure must be increased to keep them in the liquid phase.

Pervaporation, by comparison, is run at generally higher temperaturesthan perstraction and relies on vacuum on the permeate side to evaporatethe permeate from the surface of the membrane and maintain theconcentration gradient driving force which drives the separationprocess. As in perstraction, the aromatic molecules present in the feeddissolve into the membrane film, migrate through said film and emerge onthe permeate side under the influence of a concentration gradient.Pervaporative separation of aromatics from saturates can be performed ata temperature of about 25° C. for the separation of benzene from hexanebut for separation of heavier aromatic/saturate mixtures, such as heavycat naphtha, higher temperatures of at least 80° C. and higher,preferably at least 100° C. and higher, more preferably 120° C. andhigher should be used. Temperatures of about 150° C. have beensuccessfully used with polyester membranes of the present invention, themaximum upper limit being that temperature at which the membrane isphysically damaged. Vacuum on the order of 1-50 mm Hg is pulled on thepermeate side. The vacuum stream containing the permeate is cooled tocondense out the highly aromatic permeate. Condensation temperatureshould be below the dew point of the permeate at a given vacuum level.

The membrane itself may be in any convenient form utilizing anyconvenient module design. Thus, sheets of membrane material may be usedin spiral wound or plate and frame permeation cell modules. Tubes andhollow fibers of membranes may be used in bundled configurations witheither the feed or the sweep liquid (or vacuum) in the internal space ofthe tube or fiber, the other material obviously being on the other side.

When the membrane is used in a hollow fiber configuration with the feedintroduced on the exterior side of the fiber, the sweep liquid flows onthe inside of the hollow fiber to sweep away the permeated highlyaromatic species, thereby maintaining the desired concentrationgradient. The sweep liquid, along with the aromatics contained therein,is passed to separation means, typically distillation means, however, ifa sweep liquid of low enough molecular weight is used, such as liquefiedpropane or butane, the sweep liquid can be permitted to simplyevaporate, the liquid aromatics being recovered and the gaseous propaneor butane (for example) being recovered and reliquefied by applicationof pressure or lowering of temperature.

Polyester films are commercially available or can be obtained bypreparing a solution in a suitable solvent, e.g., hexafluoroacetone,hexafluoroisopropanol or trifluoroacetic acid, casting on a glass plateor a porous support, adjusting the thickness with a casting knife anddrying the membrane first at room temperature, then at high temperature,e.g., 120° C.

The present invention will be better understood by reference to thefollowing examples which are offered by way of illustration and notlimitation.

In the following examples, membranes are used to separate toluene fromisooctane in a pervaporation apparatus. The initial mixture containsapproximately equal weights of the two hydrocarbons.

The pervaporation apparatus is a cell, separated into two compartmentsby a porous metal plate, on which the membrane is supported. During apervaporation experiment the aromatics/saturates mixture is circulatedthrough the upper compartment at the desired temperature. The lowercompartment is kept at reduced pressure. The permeate is collected in atrap cooled with dry ice-acetone or dry ice-isopropanol and periodicallyanalyzed by gas chromatography.

The following examples illustrate the invention.

EXAMPLE 1

A film of polyethylene terephthalate produced by DuPont under the nameof Mylar was used in the pervaporation apparatus described above for theseparation of the mixture containing 50 wt% toluene and 50 wt%isooctane. At 170° C. the toluene/isooctane separation factor was about23, and the normalized flux was 108 Kg.μM/M² /day. At 200° C. theseparation factor was about 18 and the normalized flux was 190 Kg.μM/M²/day.

EXAMPLE 2

A film of PETG, polyethylene terephthalate/cyclohexane-dimethanolterephthalate, supplied .by Acrilex using PETG copolyester 6763manufactured by Eastman Chemical was used in the pervaporation apparatusdescribed above for the separation of the mixture containing 50 wt%toluene and 50 wt% isooctane. The FIGURE shows the toluene/isooctaneselectivity along with the permeability for the PETG membrane as afunction of temperature. As shown in this FIGURE, this membrane had ahigh selectivity of about 50 and a permeability of about 300 Kg.μM/M²/day at 100° C. The selectivity decreased whereas the permeabilityincreased with increasing temperature. At 150° C., this membrane stillhad a good selectivity of about 12 and a permeability of about 1300Kg.μM/M² /day.

What is claimed is:
 1. A method for separating mixtures of aromatics andnon-aromatics into aromatic-enriched and non-aromatic-enriched streamscomprising:(a) contacting said aromatics/non-aromatics mixture with oneside of a membrane consisting essentially of a polyester, and (b)selectively permeating the aromatic components of the mixture throughthe membrane.
 2. The method of claim 1 wherein said membrane ispolyethylene terephthalate.
 3. The method of claim 1 wherein saidmembrane is polyethylene terephthalate/cyclohexane-dimethanolterephthalate.